Spray method for forming shells for prostheses

ABSTRACT

Shells for mammary prostheses and other devices are created by spraying a silicone dispersion onto a mandrel. Several coats of dispersion are applied with an interval for evaporation of solvent from the dispersion between application of coats. The shells created are uniform in thickness and have a desirably defect-free surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.10/918,277, filed on Aug. 13, 2004.

TECHNICAL FIELD

This invention relates to devices that are implantable in the human bodysuch as prostheses and tissue expanders.

BACKGROUND

Implantable prostheses are commonly used to replace or augment bodytissue. Such prosthesis include a shell formed of an elastomericmaterial, e.g., silicone. The shell is filled with filling material suchas saline or some other liquid or a gel. The filling of the shellcommonly takes place after the shell is inserted through an incision.The shell includes a valve that can accept a filling tube that passesthrough the incision and is used to fill the shell with a suitableliquid or gel. Once the shell is filled to the desired degree, thefilling tube is removed and the incision is closed.

In the case of the female breast, it sometimes necessary to remove someor all of the mammary gland and surrounding tissue in order to treatbreast cancer. This surgery leaves a void that can be filled with animplantable prosthesis. The implant serves to support surrounding tissueand to maintain the appearance of the body. The restoration of thenormal appearance of the body has an extremely beneficial psychologicaleffect on post-operative patients, eliminating much of the shock anddepression that often follows extensive surgical procedures.

Implantable prostheses are also used more generally for restoring thenormal appearance of soft tissue in various areas of the body.

Tissue expanders generally resemble implantable prostheses except thatthey include a means for adding additional liquid or gel after thedevice had been inserted under the skin and the incision has beenclosed. After implantation the shell is gradually inflated using aliquid or gel, usually over a period of weeks, in order to expand theoverlying skin either so that a prosthesis can later be inserted or sothat skin can be generated for grafting. The liquid or gel is usuallyintroduced by means of a needle that pierces the skin and a self-sealvalve that is integral to the shell or that is remote from the shell andconnected to the shell by tubing.

Silicone shells for implantable prosthesis and tissue expanders aregenerally formed by dipping a suitably shaped mandrel into a siliconedispersion. The mandrel is withdrawn from the dispersion and the excesssilicone dispersion is allowed to drain from the mandrel. After theexcess dispersion has drained from the mandrel at least a portion of thesolvent is allowed to evaporate to stabilize the silicone coating. Theprocess is then repeated one or more times until a shell of the desiredthickness is formed. Because the flow of the silicone dispersion as itdrains from the mandrel depends on the shape and orientation of themandrel, the resulting shell can vary substantially in thickness. Inaddition, because dip casting requires a relatively large vat ofsilicone dispersion and because solvent evaporates from the siliconedispersion in the vat during the casting process, considerable siliconedispersion waste is created during dip casting.

SUMMARY

The invention features methods for forming the shell of a body implant(e.g., a prosthesis or tissue expander). The methods can also be used tomake other articles formed from an elastomeric material (e.g., a patchfor an implant) and for applying a coating of an elastomeric material(e.g., silicone) to a device to be implanted into the body (e.g., apacemaker or implantable pump). The methods of the invention entailspraying a silicone dispersion onto a mandrel or other object. Thesilicone dispersion is sprayed using, for example, a high volume, lowpressure (HVLP) spray device or a rotary atomizer or some other devicethat sprays the silicone dispersion at a low pressure. The methodsdescribed herein can be used, for example, to create an implant shellthat is both thinner and more uniform in thickness than that which canbe formed using traditional dip casting techniques. In addition, themethods of the invention can be used to create a shell that varies inthickness in a controlled manner. Further, the methods of the inventionallow the production of shells having complex shapes that cannot beefficiently formed using traditional dip casting methods. Thus, themethods can be used to make generally spherical, cylindrical,rectangular solids, and cubic shell as well as other shapes, includingshapes having edges, corners, recessed regions and other complexgeometries. Shells created by the methods of the invention have agenerally smoother surface than shells created by traditional dipcasting methods.

In one aspect the invention features a method comprising: a) providing amandrel; b) spraying a silicone dispersion onto the mandrel until adesired thickness of silicone dispersion is formed on the mandrel; c)curing the silicone dispersion to form a silicone device; and d)removing the silicone device from the mandrel. In various embodiments:the silicone dispersion comprises high temperature vulcanization (HTV)silicone, the silicone dispersion comprises room temperaturevulcanization (RTV) silicone.

In one aspect the invention features a method for creating a siliconeshell, the method comprising: a) providing a mandrel; b) applying acoating silicone dispersion to the mandrel by spraying the silicondispersion at low pressure; c) allowing evaporation of at least aportion of the solvent in the coating silicone dispersion; d) repeatingsteps b) and c) until a silicone shell having a desired thickness isformed; e) at least partially curing the silicone shell; and f) removingthe silicone shell from the mandrel.

In various embodiments: the silicone dispersion is sprayed onto themandrel using a high volume low pressure spray device; the siliconedispersion is sprayed onto the mandrel using a rotary atomizer; thesilicone dispersion is an HTV silicone dispersion; the siliconedispersion is an RTV silicone dispersion; at least two coats ofdispersion are applied to the mandrel; at least three coats ofdispersion are applied to the mandrel; at least four coats of dispersionare applied to the mandrel; at least five coats of dispersion areapplied to the mandrel; at least six coats of dispersion are applied tothe mandrel; the dispersion is sprayed by atomizing the dispersion usinga flow of air below 20 psi; the dispersion is sprayed by atomizing thedispersion using a flow of air below 10 psi; the shell is 0.00±0.004″thick; the shell is 0.012″±0.004″ thick; the shell is 0.014±0.004″thick; and the shell is 0.013″±0.004″ thick.

In other embodiments, the method further comprising applying a coat ofsilicone dispersion to a portion of the mandrel to create a partial coatof dispersion; the mandrel includes an anterior surface and a posteriorsurface that meet at a perimeter region; the partial coat of dispersionis applied to the perimeter region of the mandrel; the mandrel has atleast one relatively planar region and at least on region that iscurved; the partial coat of dispersion is applied to the at least oneregion that is curved; the mandrel has a first region having a firstradius of curvature and a second region having a second smaller radiusof curvature; the region having the partial coat of dispersion isapplied to the region having a second, smaller radius of curvature; andat least one partial coat of dispersion is applied to at least a firstportion of the mandrel.

The invention also features a shell for a tissue expander formed byabove-described method and a shell for an implantable prosthesis made bythe above-described method.

The invention also features a prosthesis formed by a method comprisingproviding a shell formed by the above-described method and sealing theshell or sealing the shell while providing the shell with a fillingport.

For RTV silicone the silicone dispersion comprises 20-70% siliconesolids, 20-60% silicone solids, 20-50% silicone solids, 25-45% siliconesolids, 28-40% silicone solids, 28-39% silicone solids, 28-38% siliconesolids, 28-36% silicone solids, or 28-34% silicone solids. In certainembodiments the RTV dispersion contains 30-35% solids, preferably31%+/−3% silicone solids, 31%+/−2% silicone solids, or 31%+/−1% siliconesolids. In some embodiments the dispersion contains xylene or anothersuitable solvent.

For HTV silicone the silicone dispersion comprises 20-70% siliconesolids, 20-60% silicone solids, 20-50% silicone solids, 25-45% siliconesolids, 30-38% silicone solids, or 32-36% silicone solids. In certainembodiments the HTV dispersion contains 34%30 /−3% silicone solids,34%+/−2% silicone solids, or 34%+/−1% silicone solids. In someembodiments the dispersion contains xylene or another suitable solvent.

For products filled with saline, the shell is formed of RTV silicone insome embodiments and is formed using a dispersion having a viscosity of700 to 820 centipoise. For products filled with silicone gel, the shellis formed of HTV silicone in some embodiments and is formed using adispersion having a viscosity of 500 to 600 centipoise.

The invention also features a silicone device formed by: a) providing amandrel; b) spraying a silicone dispersion onto the mandrel until acoating of the desired thickness is formed on the mandrel; c) curing thecoating of silicone dispersion on; and d) removing the silicone devicefrom the mandrel.

The method can be used to form an HTV silicone shell with a thicknessof: 0.013″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″, ±0.0025″ or±0.002″; 0.012″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″, ±0.0025″ or±0.002″; 0.011″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″, ±0.0025″ or±0.002″; 0.010″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″, ±0.0025″ or±0.002″; or 0.009″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″, ±0.0025″or ±0.002″. In certain embodiments the method is used to create asilicone shell that varies in thickness by no more than 0.006″, 0.005″,0.004″, 0.003″, 0.0025″ or 0.002″.

The method of the invention is used to form an RTV shell with athickness of: 0.015″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; 0.014″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; 0.013″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; 0.012″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; 0.011″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; 0.010″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; or 0.009″±0.007″, ±0.006″, ±0.005″, ±0.004″,±0.003″, ±0.0025″ or ±0.002″. In certain embodiments the method is usedto create a silicone shell that varies in thickness by no more than0.006″, 0.005″, 0.004″, 0.003″, 0.0025″ or 0.002″.

In some embodiments the mandrel is sprayed with 3, 4, 5, 6, 7 or morecoats of silicone dispersion. In some embodiments solvent is allowed toevaporate from the dispersion for 1, 2, 3, 4, 5, 10, 15, 20 or moreminutes between coats. In some embodiments the mandrel is rotated duringapplication of the one or more coats of silicone dispersion.

In certain embodiments the first coat of dispersion on the mandrelcontains less dispersion than subsequent coats. For example, the firstcoat is formed by two passes of the spray head over each portion of themandrel and subsequent coats are formed by three passes of the sprayhead over each portion of the mandrel. Thus, the first coat containsless material that subsequent coats and is generally thinner thansubsequent coats.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of the anterior face of the shell of a mammaryprothesis.

FIG. 2 is schematic drawing of a cross-section of the shell of a mammaryprosthesis taken along A-A′ in FIG. 1.

DETAILED DESCRIPTION

Described below are methods for forming a silicone articles, e.g., ashell for soft tissue prostheses. The methods entail spraying a siliconedispersion onto a suitably shaped mandrel or an object to be coatedusing, for example, a high volume, low pressure (HVLP) spray device or arotary atomizer. The methods of the invention allow one to control thethickness of shell, permitting the creation of shells that are of auniform thickness as well as shells that vary in thickness in acontrolled manner.

The spraying of the silicone dispersion can be accomplished using HVLPsystems. The spraying can be accomplished by atomizing the dispersionusing relatively low pressure air. The spraying can also be accomplishedby rotary atomization. In rotary atomization the dispersion is feed to acone or bell spinning at, e.g., 10,000-40,000 RPM. The dispersion isfeed to the cone or bell at low pressure (e.g., the dispersion pressureis at or below 30 psi, below 20 psi, 10±4 psi or between 0.1 and 10psi). Rotary atomization forms a very smooth, uniform layer of siliconethat has very few pits or other imperfections. Rotary atomizers aredescribed well known in the art (see, e.g., U.S. Pat. Nos. 5,633,306 and4,887,770). Rotary atomizers are available from Ransburg Corporation(Toledo, Ohio) and other suppliers. The spray head (cone or bell) of arotary atomizer can be controlled by a programmable computer so that thespray head makes the same number of passes over all portions of themandrel or so that the spray head makes more passes over some portionsof the mandrel than other portions.

The methods of the invention have numerous advantages over thetraditional dip casting technique used to create shells for soft tissueimplants. In dip casting, a shell is formed by first dipping a mandrelinto a silicone dispersion. Once the mandrel is removed from thedispersion, excess dispersion is allowed to drain from the mandrel.Solvent is allowed to evaporate from the coating of silicone dispersionremaining on the mandrel until the coating is sufficiently stabilized toallow the mandrel to be dipped into the dispersion again. This processis repeated until a shell of the desired thickness is created. Becausethe flow of the silicone dispersion as it drains from the mandreldepends on the shape and orientation of the mandrel, the resulting shellcan vary substantially in thickness. For example, the thickness of theshell of a mammary prosthesis formed by dip casting might vary from0.009 to 0.024″ from one region to another, a variation of more than100%. Importantly, the thickness of the various regions of the shell isdictated for the most part by the shape of the mandrel and theorientation of the mandrel during the period that the siliconedispersion is draining from the mandrel.

FIG. 1 is a plan view of a mammary prosthesis. The anterior face 100 isthe outward face when the prosthesis is inserted under the skin of apatient's chest. The prosthesis includes an upper pole region 120 (i.e.,the upper half of the shell when the prosthesis recipient is standing)and a lower pole region 130 (i.e., the lower half of the shell when theprosthesis recipient is standing). The region 135 where the anteriorface 100 meets the posterior face (not shown) is sometime referred to asthe radius, perimeter or edge

FIG. 2 is a cross-sectional view of a mammary prosthesis taken alongA-A′ in FIG. 1. This type of shell has a shape that resembles that of ahuman breast in that it is fuller in the lower portion than in the upperportion. The shell has an anterior face region 100, a posterior faceregion 110 (i.e., the face placed against the patient's chest wall whenthe prosthesis is implanted), an upper pole region 120 and a lower poleregion. The region where the posterior and anterior face or surface meet135, sometimes referred to as the radius, perimeter or edge, isrelatively curved compared to the posterior face and to some extent eventhe anterior face. In the shell depicted here, the radius of curvatureof the perimeter in the upper pole region is relatively small. Theradius of curvature of the perimeter in the lower pole region is larger.As noted above the shell depicted in FIG. 2 has a shape that moreclosely resembles the human breast than other types of shells, e.g.,shells that are symmetrically dome shaped.

In dip casting the mandrel is held such that the portion correspondingto the anterior face of the shell faces downward. The bracket or rodused to hold the mandrel as it is dipped into the dispersion extendsoutward from that portion of the mandrel corresponding to the posteriorface of the shell. As dispersion drains from the mandrel after it iswithdrawn from the dispersion it can be appreciated that it drainsrelatively rapidly from the perimeter region. As a result, the shell inthe perimeter region tends to be thinner than the shell in the anteriorface region. For this reason, additional dips are required to create anadequate shell thickness in the perimeter region. This can result inshell that is thicker than desired in the anterior face region. Inaddition, because the dispersion flows over the mandrel and onto theportion corresponding to the posterior face of the shell, the posteriorface region is generally thicker than the shell anterior face region andcan be thicker than desired.

As can be seen, the radius of curvature in upper pole transition region180 between the anterior face region 100 and posterior face region 110is quite small. It can be desirable for the shell in this region to bethicker than, for example, the anterior face region 100, which should berelatively thin to preserve a natural appearance when implanted in apatient. The method of the invention allows for creating a thicker shellin upper pole perimeter or radius region while maintaining a thinnershell in other regions, e.g., the anterior face region 100 by simplycontrolling the spray head to apply additional coats of siliconedispersion in the upper pole perimeter or radius region.

In contrast, the methods of the invention can be used to create a shellfor a mammary implant or other that varies in thickness from, e.g.,0.009″±0.003″ on the faces (e.g., the anterior and posterior faces) to0.024″±0.003 on the edges or corners where two or more faces meet. Themethod can also be used to create shells with greater variation inthickness. Thus, as described in greater detail below, the methods ofthe invention can also be used to create shells that vary in diameter ina controlled manner, i.e., a manner that is not dictated by the shape ofthe mandrel.

In one embodiment of the invention, a silicone dispersion is applied toa mandrel using a robotically controlled rotary atomizer spray head. Therotary atomizer can operate electrostatically, wherein there is adifference in electrical potential between the silicone and the mandrel.For example, a charge can be applied to the dispersion and the mandrelcan be grounded, such that the atomized silicone dispersion is attractedthrough electrical forces to the mandrel. The spray head makes one ormore (e.g., 1, 2, 3, 4, 5, or more) passes over an area of the mandrelto apply one coat of silicone to the entire mandrel or desired portionthereof. The spray head or the mandrel or both can be moved during theapplication of a coat of silicone dispersion. A number of coats areapplied depending on various factors such as the type of silicone used(HTV or RTV), the percent of silicone solids in the dispersion, and thedesired thickness of the shell. Between the application of each coat ofsilicone dispersion solvent is allowed to evaporate so that the coat ofsilicone dispersion is at least somewhat stabilized prior to theapplication of the next coat of silicone dispersion. Thus, there is aninterval of several minutes (e.g., 2, 3, 4, 5, 6, 10, 12, 15, 20. 25minutes or more) between the application of one coat of dispersion(which may require 1, 2, 3, 4 or more passes of the spray head over allor a portion of the mandrel) and the next coat of dispersion. Once thedesired number of coats of silicone dispersion have been applied, theshell is allowed to fully cure and it is then removed from the mandrel.Because there is no need for excess dispersion to drain from themandrel, each coat applied is quite uniform in thickness, i.e., it isnot significantly thicker in the posterior face region than it is in theanterior face region.

In applying the dispersion, some portions of the mandrel can receivemore coats of dispersion than other portions of the mandrel. Forexample, in the case of a mammary implant the perimeter region canreceive more coats of dispersion than the anterior face. In general itcan be desirable to apply more coats of material (resulting in a thickershell) in those regions that correspond to an edge or corner or thatotherwise have a smaller radius of curvature than in those regions thatare relatively planar.

To provide shells or other devices with one or more regions that arethicker than one or more other regions, it can be desirable to applymore coats of silicone dispersion to some portions of the mandrel thanother portions. Thus, one can apply a partial coat of dispersion, i.e. acoat that does not cover the entire mandrel, but instead covers onlyportion of the mandrel. Additional partial coats can be applied to themandrel so that one or more portions of the mandrel have 1, 2, 3, 4 ormore additional coats of dispersion compared to other portions of themandrel. Moreover, additional partial coats can be applied to two ormore different regions of the mandrel. Thus, the entire mandrel canreceive a total 4 coats of dispersion, one portion can receive a totalof 5 coats of dispersion and yet another portion can receive a total of6 coats of dispersion.

It is desirable to use a silicone dispersion having a defined percentsolids so that the thickness of the layer of silicone dispersion appliedin each coat is relatively predictable. This is in contrast totraditional dip casting methods in which it is desirable to use asilicone dispersion having a defined viscosity. A silicone dispersionhaving a defined viscosity is desirable in dip casting because theformation of the shell is highly dependent on the bulk flowcharacteristics of the silicone dispersion. In many instances therelationship between viscosity and percent solids is not predictable.Thus, for shells produced by spray techniques it is desirable to use adispersion with a defined percent solids rather than a definedviscosity.

EXAMPLE 1 RTV Silicone Shell

An appropriately shaped mandrel for forming the shell of a mammaryprosthesis is arranged approximately 3″ to 6″ inches from a roboticallycontrolled spray head of rotary atomizer spray device (e.g., theAerobell RMA-101; ITW Ransburg, Inc.; Toledo Ohio). The mandrel isarranged so that the portion of the mandrel corresponding to theposterior face of the shell is facing downward. The mandrel is held on arod that extends from the downward facing portion of the mandrel andthis rod is arranged so as to allow the mandrel to be rotated on theaxis of the rod. The mandrel and the spray head are contained in acabinet where the air temperature is held at 90° F.±10° F. and about35-45% relative humidity. A RTV silicone dispersion having 31% (±2%)solids in a xylene dispersion is applied to the mandrel using a sprayhead that travels in an arc from above the mandrel (90° above thehorizontal) to below the mandrel (90° below horizontal) completing onepass from above the mandrel (the portion of the mandrel corresponding tothe anterior face of the shell) to below the mandrel (the portion of themandrel corresponding to the posterior face of the shell) in about 5seconds as the mandrel is rotated at about 20 rpm. The spray head issupplied by a ⅜″ diameter supply line and the dispersion is pumpedthrough the line at 8-20 p.s.i. The dispersion can be electricallycharged and the mandrel grounded in order to electrostatically attractthe silicone dispersion to the mandrel. However, since RTV siliconedispersion is not particularly conductive, similar results can beachieved with and without charging the dispersion. To apply one coat,the spray head travels from above the mandrel to below the mandrel andreturns to above the mandrel during which time the mandrel rotates about5 full revolutions. The application of a single coat of dispersion takesabout 15 seconds. Solvent is allowed to evaporate from the siliconedispersion coated on the mandrel for 10 to 20 minutes in adevolatilization step. This process is repeated so that 4 to 5additional coats of dispersion are applied in the same manner with adevolatilization step occurring between each coat. The final thicknessof the shell is 0.014″±0.002″. The process can be automated by mountinga number of mandrels on a track that passes the mandrels by the sprayhead. The spray head can move with the line or the line can pause with amandrel positioned near the spray head. After the final coat has beenapplied, the shell is cured by placing the coated mandrel in an oven setto 150° F. for at least 20 to 30 minutes. The shell is stripped from themandrel and can be used in a tissue expander or prosthesis.

EXAMPLE 2 HTV Silicone Shell

An appropriately shaped mandrel for forming the shell of a mammaryprosthesis is arranged approximately 3″ to 6″ inches from a roboticallycontrolled spray head of rotary atomizer spray device (e.g., theAerobell RMA-101; ITW Ransburg, Inc.; Toledo Ohio). The mandrel isarranged so that the portion of the mandrel corresponding to theposterior face of the shell is facing downward. The mandrel is held on arod that extends from the downward facing portion of the mandrel andthis rod is arranged so as to allow the mandrel to be rotated on theaxis of the rod. The mandrel is heated to about 145° F. by an infraredheating device that allows the surrounding air temperature to besignificantly lower. The mandrel and the spray head are contained in acabinet where the air temperature is held at 115° F.±10° F. and about35-45% relative humidity. A HTV silicone dispersion having 31% (±2%)solids in a xylene dispersion is applied to the mandrel using a sprayhead that travels in an arc from above the mandrel (90° above thehorizontal) to below the mandrel (90° below horizontal) completing onepass from above the mandrel (the portion of the mandrel corresponding tothe anterior face of the shell) to below the mandrel (the portion of themandrel corresponding to the posterior face of the shell) in about 5seconds as the mandrel is rotated at about 20 rpm. The spray head issupplied by a ⅜″ diameter supply line and the dispersion is pumpedthrough the line at 8-20 psi. Shaping air can be employed to shape thestream of silicone dispersion. The dispersion can be electricallycharged (e.g., by applying 80,00 to 100,000 volts at low amperage) andthe mandrel grounded in order to electrostatically attract the siliconedispersion to the mandrel. Because HTV silicone dispersion isconductive, superior results can be achieved by charging the dispersion.To apply one coat, the spray head travels from above the mandrel tobelow the mandrel and returns to above the mandrel during which time themandrel rotates about 5 full revolutions. The application of a singlecoat of dispersion takes about 15 seconds. Solvent is allowed toevaporate from the silicone dispersion coated on the mandrel for 10 to20 minutes in a devolatilization step. This process is repeated so that3 to 4 additional coats of dispersion are applied in the same mannerwith a devolatilization step occurring between each coat. The finalthickness of the shell is 0.012″±0.002″. The process can be automated bymounting a number of mandrels on a track that passes the mandrels by thespray head. The spray head can move with the line or the line can pausewith a mandrel positioned near the spray head. After the final coat hasbeen applied, the shell is cured by placing the coated mandrel in anoven set to 325° F. for at least 55 to 65 minutes. The shell is strippedfrom the mandrel and can be used in a tissue expander or prosthesis.

EXAMPLE 3 Textured Shell

A textured shell can be created by applying solid particles of siliconeto a shell between coats of silicone dispersion. A shell of the desiredthickness is created as described in Example 1 or Example 2. The shellis partially or fully cured by heating. Alternatively, the shell is notcured at all. The tackiness of an uncured or partially cured siliconeshell fosters adhesion of the solid particles. Particles of fully curedHTV or RTV silicone having an average diameter of 100 to 600 microns areapplied to the surface of the shell, e.g., by dipping the mandrel into abed of particles or by blowing particles onto the shell. Because thesurface of the shell is tacky and because the silicone particles have astatic charge, the silicone particles adhere readily. The siliconeparticles can be applied at a density that allows almost completecoverage of the shell or partial coverage. Thus, the particles can beapplied relatively densely so that there is little exposed shell or theycan be applied relatively sparsely so that there is considerable exposedshell. In addition the particles can be applied to only a portion of theshell, e.g., the particles can be applied only to the posterior face ofthe shell. The shell is heated (at 250-325° F. for 30 to 60 minutes inthe case of HTV silicone) to partially cure or gel the silicone layer towhich the particles are adhered. Particles that do not adhere to theshell are removed by gently blowing air over the surface of the shell.The shell is then sprayed again with a silicone dispersion (e.g., HTVsilicone diluted to, e.g., 10 to 13% solids with xylene, toluene,tetrahydeofuran or some other suitable solvent) to apply a particlecoating layer that envelops the applied particles. The particle coatinglayer is partially cured or gelled by, for example, heating the shell to250-325° F. for 30 to 60 minutes in the case of particles coated with anHTV silicone dispersion. The application of particles, curing, coatingwith silicone dispersion and curing is repeated two more times for atotal of three application of particles and three applications ofsilicone dispersion. Finally, the shell is fully cured.

Silicone particles can be created, for example, from fully curedsilicone that is cryogenically ground to yield particles ranging in sizefrom 100-600 microns in diameter, e.g., 100-200, 200-300, 300-400,400-500, or 500-600 microns in diameter. For example, ¼ to ½ inch thicksilicone sheets can be cryogenically ground to yield suitable particles.In some cases the particles are subsequently size selected, e.g., bysieving, such that the particles have a selected average size, e.g.,200-300 microns. In some cases the size selection is limited to removingvery small particles and very large particles.

As noted above, the silicone particles can be applied several times. Insome cases that average size of the silicone particles applied willvary. Thus, the particles applied in the first application of particlescan have a first average size and the particles applied in the secondapplication of particles can have a second average size. If there are isa third applications of particles, the particles in this thirdapplication can have a third average particle size. Thus, the particlesused in each application can have a different average particle size. Insome cases, several of the applications of particles can have the sameor very similar average particle size. In some cases a given applicationof particles can include two different groups of particles, one having afirst average particle size and the other having a second, differentaverage particle size.

The surface created on the shell is microporous. There are cavities,overhangs, bridges and passageways. However, because each layer ofparticles is enveloped in silicone, the surface is relatively smoothlymodeled. Because the surface of the shell includes cavities, overhangsand passageways, upon implantation, tissue will grow into the cavities,beneath the overhangs and through the passageways. In this manner, thetissue is engaged with the implant. This engagement secures the implant.

OTHER EMBODIMENTS

A soft tissue prosthesis can have any desired shape, e.g., the shell ofthe prosthesis can be circular, oval, or crescent shaped. The prosthesiscan have a single lumen or multiple lumens. It can be formed of siliconerubber, a laminate of various forms of silicone, silicone copolymers,polyurethane, and various other elastomers in various combinations.Various materials are described in U.S. Pat. Nos. 4,592,755 and4,205,401.

To form a prosthesis from the shell, e.g., a shell formed of HTVsilicone, the opening in the posterior face of the shell is sealed usinga patch comprising a vulcanized layer of silicone sheeting and anunvulcanized layer of silicone. The patch is shaped and sized to besomewhat larger than the opening in the posterior face of the shell. Thepatch is positioned inside the shell such that the unvulcanized layer ofthe patch faces outward and the perimeter of the patch overlaps the edgeof the shell surrounding the opening. The assembly is compressed eitherbetween hot platens at, e.g., 325° F. and 60 p.s.i. or platens at roomtemperature and 60 p.s.i. for about two to three minutes. The patchedshell is then cured in an oven at 325° F. for about one half hour tocure fully.

The shell can be filled with a fluid or gel. In addition, an amount ofsolid material can be combined with the fluid or gel to adjust thedensity or compressibility of the filling.

Elastomers other than silicone may be used. Thus, the mandrel can besprayed with a dispersion of any elastomer.

The prosthesis of the invention can be provided as a kit with a shelland a means for filling the shell, e.g., a syringe. The kit can furtherinclude an adapter tube for connecting the syringe to the filling portof the shell.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.

1. A shell for an implantable prosthesis formed by: a) providing amandrel; b) applying a x silicone dispersion coating having aconcentration of silicone to solvent of greater than 20 percent to themandrel by spraying the silicone dispersion at low pressure; c) allowingevaporation of at least a portion of the solvent in the coating siliconedispersion; d) repeating steps b) and c) until a silicone shell having adesired thickness is formed; e) at least partially curing the siliconeshell; and f) removing the silicone shell from the mandrel.
 2. The shellaccording to claim 1, wherein the shell is sized and shaped for use as amammary prosthesis.
 3. The shell according to claim 2, wherein at leasta portion of an external surface of the shell is textured.
 4. The shellaccording to claim 3, wherein the shell is sealed.
 5. The shellaccording to claim 1, and further formed by, following the removingstep, sealing the shell while providing the shell with a filling port.6. The shell according to claim 5, wherein the shell is sized and shapedfor use as a mammary tissue expander.
 7. The shell according to claim 6,wherein at least a portion of an external surface of the shell istextured.
 8. A mammary prosthesis formed by: a) providing a mandrel; b)applying a silicone dispersion coating having a concentration ofsilicone to solvent of greater than 20 percent to the mandrel byspraying the silicone dispersion at low pressure; c) allowingevaporation of at least a portion of the solvent in the coating siliconedispersion; d) repeating steps b) and c) until a silicone shell having adesired thickness is formed; e) at least partially curing the siliconeshell; and f) removing the silicone shell from the mandrel.
 9. Theprosthesis according to claim 8, wherein the prosthesis is adapted to befilled with fluid, and wherein when so filled the prosthesis is sizedand shaped for use as a mammary implant.
 10. The prosthesis according toclaim 9, wherein at least a portion of an external surface of theprosthesis is textured.
 11. The prosthesis according to claim 10,wherein the prosthesis is sealed.
 12. The prosthesis according to claim8, and further formed by, following the removing step, sealing theprosthesis while providing the prosthesis with a filling port.
 13. Theprosthesis according to claim 12, wherein the prosthesis is sized andshaped for use as a mammary implant.
 14. The prosthesis according toclaim 13, wherein at least a portion of an external surface of theprosthesis is textured.